In many areas of the United States natural gas shortages are threatening to strangle industry to a degree that could be much more severe than the widely publicized Arab oil embargo. For example, this winter of 1974-1975 in many midwestern states industrial users will receive only about one-half of last year's allotment of natural gas. Unfortunately, according to the most credible projections available, the natural gas supply situation will not improve. Therefore, for the intermediate and long term, synthesis gas, hereinafter SNG, will have to play a larger role if anything near our present industrial and general life style is to be maintained.
However, for SNG to provide a significant portion of our total gas needs great amounts of capital will have to be made available that would otherwise be used for alternate purposes, requiring much higher costs to the consumer.
To reduce the impact of an SNG industry on our fuel costs will require the development of technology that allows lowering capital and operating costs substantially below that required for the current and heretofore proposed systems for coal gasification. The present invention comprises a method of treating coal which permits conversion of coal to SNG under previously unobtainable conditions that allow substantial reductions to be made in plant investment and operating costs.
Work on coal gasification process development has been going on for years. For example, the Lurgi process was first operated commercially in 1936 and the Winkler process was used on a commercial scale in the 1920's. However, commercialization of synthesis gas-from-coal processes never became important in the U.S. because of the large Texas gas and oil fields coming into production shortly after World War II.
It is well recognized that coal gasification technology could benefit considerably by the development of suitable coal gasification catalysts. Numerous attempts have been made since the beginning of this century to catalyze the reaction of coal and other carbonaceous matter with steam. A few attempts have also been made recently to catalyze the reaction of coal and other carbonaceous matter with hydrogen, hereinafter termed hydrogasification, because of the increased interest in producing methane from coal.
In the 1920's Taylor and Neville reported data on the effect of several catalysts on the steam-carbon reaction at 490.degree.-570.degree. C showing that the most effective catalysts were potassium and sodium carbonate, and Kroger found that metallic oxides and alkali carbonates or mixtures catalyzed the steam-carbon reaction.
While the catalytic and noncatalytic steam-carbon reactions had been studied in fair detail before 1940, little had been studied on the reaction of carbon with hydrogen. In 1937, Dent was the first to report on methane formation by the reaction of hydrogen with coke and coal, hydrogasification, at elevated temperatures and pressures. Dent's work did not involve the use of a catalyst.
Several studies have been conducted since 1960 on the catalysis of hydrogasification reactions involving carbonaceous matter and various oxidizing and reducing gases. Wood and Hill reported that the hydrogasification of coals and cokes at 800.degree.-900.degree. C is catalyzed by 1-10 weight percent alkali carbonates. The increased hydrogasification rates have been attributed to the prevention of graphitization of the reaction surface due to adsorption of alkalies. Le Francois has recently described a process that uses molten sodium carbonate as a catalyst for the steam-coal reaction. Very high ratios of molten salt to coal are required since the molten salt is the continuous phase.
Haynes, Gasior, and Forney have been working on the high-pressure catalytic gasification of coal with steam. In their bench-scale experiments at 850.degree. C and 300 psig they found that alkali metal compounds increased the carbon gasification the most, by 31-66 percent. The catalyst concentration was 5 weight percent of coal in all cases. The coal was high-volatile bituminous coal (Bruceton, Pennsylvania) that had been pretreated at 450.degree. C with a steam-air mixture to make it noncaking. They also found that 20 different metal oxides, including CaO, increased carbon gasification by 20-30 percent.
The latter works conducted some pilot plant experiments in the Synthane gasifier at 907.degree.-945.degree. C and 40 atmospheres, and found that a 5-weight percent "addition" to the coal of either dolomite or hydrated lime resulted in significant increases in the amount of carbon gasified and in the amount of CH.sub.4, CO, and H.sub.2 produced.
In all of the above-described prior art only two methods for impregnation of coal with a catalyst have been used: (a) physical admixing of catalyst to coal, or (b) soaking of coal in an aqueous solution of catalyst at room temperature and then drying the slurry.
The present invention involves the chemical and physical incorporation of a suitable gasification catalyst in coal by hydrothermally treating the coal. Gasification tests of coal treated according to the present invention indicate that this coal has a reactivity far above that predictable from the results of the investigations described above. Coal treated according to the present invention is a much better feedstock for gasification than either raw coal or coal impregnated with comparable quantities of catalysts according to the prior art.
The following are the improved characteristics of coal treated according to the present invention, which can result in a number of advantages:
(1) A highly caking and swelling coal can be made completely non-caking and non-swelling without any significant loss of the volatile matter. This should result in (a) simpler reactor systems, (b) higher system reliability, and (c) more efficient coal utilization. (2) Hydrogasification of HTT coal proceeds at lower pressures which should result in (a) lowering of the investment cost and (b) higher system reliability. (3) Hydrogasification of HTT coal proceeds at higher rates which should result in (a) high direct yield of methane, (b) a compact reactor, and (c) in simplified gas purification.
(4) Steam gasification of HTT coal proceeds at a lower temperature which should result in (a) lower oxygen consumption for gasification, (b) increased methane formation, and (c) simpler gasifiers with reduced refractory problems.
(5) If one of the catalysts in HTT coal is calcium (or magnesium) oxide it acts as an efficient absorber of sulfur in coal which should allow the combustion of the char, from gasification, without stack gas scrubbing and should result in a reduced cost for the purification of the synthesis gas.
These advantages will result in the following benefits to the gas production industry:
(1) Reduced capital investment because of the lower pressure at which direct hydrogasification occurs as well as the simpler reactor systems possible.
(2) Reduced operating costs because of the lower oxygen consumption, more efficient coal utilization, and higher system reliability.
(3) Reduced time required to bring SNG plants on stream. Because of the lower operating pressure, steel plate availability will be higher, fabrication will be faster, and quicker deliveries can be anticipated for auxiliary plant equipment.
(4) Even the most highly caking eastern coals containing high levels of sulfur can be used, thereby resulting in a considerable reduction in the SNG transportation costs and allowing the utilization of coal that could not otherwise be used.
Coal is the major source of energy for the United States and will continue to be for many years. However, one of the problems with coal as the source of energy is its high sulfur, nitrogen, and ash content which includes significant quantities of toxic (hazardous) impurities such as mercury, beryllium, and arsenic. These materials find their way into the environment during the combustion of coal and thus constitute a health hazard through atmospheric and food chain consumption.
The three different classes of impurities--sulfur, nitrogen, and metal values--are found in coal in a variety of forms.
Sulfur occurs in coal chiefly in three forms: (1) inorganic, (2) sulfate, and (3) organic. A fourth form, elemental sulfur, is rare. Of the inorganic sulfur compounds, iron pyrite (FeS.sub.2 with an isometric crystal form) and marcasite (FeS.sub.2 with the orthorhombic crystal form) are the most common. Other inorganic sulfides, chalcopyrite-CuFeS.sub.2, arsenopyrite-FeAsS, and stibnite-Sb.sub.2 S.sub.3, have been found, but they are rare.
Of the two major inorganic sulfides, pyrite is the most common. It is found in coal as macroscopic and microscopic particles, as discrete grains, cavity fillings, fiber bundles, and agregates. The concentration of pyritic sulfur vary widely even within the same deposit. Normally, the concentration will vary from 0.2 to 3 percent (sulfur basis), depending on the location.
The most common sulfur is calcium sulfate. Sulfates of iron, copper, and magnesium may also occur, but they are not abundant. Normally coal contains less than 0.1 percent sulfate sulfur, although in heavily weathered coal it may be as much as 1 percent. Because of its normally low concentration it is of little concern in air pollution.
The third form of sulfur most prevalent in coal is organic sulfur. Since this sulfur is part of and is linked to the coal itself, positive identification of the organic sulfur compounds has not been possible. However, it is usually assumed that organic sulfur is in one of the following forms:
(1) Mercaptan or thiol, RSH PA1 (2) sulfide or thio-ether RSR' PA1 (3) disulfide, RSSR' PA1 (4) aromatic systems containing the thiophene ring.
The sulfur could be present as .delta.-thiopyrone.
No definite relationship between the organic and pyritic sulfur contents of coal has been established. In typical U.S. coal, the organic sulfur may range from 20.8 to 83.6 percent of total sulfur and have a mean value of 51.2 percent of the total sulfur. The variation of the organic sulfur content of a coal bed from top to bottom is usually small. Pyritic sulfur content may vary greatly.
Nitrogen, like sulfur, is probably part of and linked to the coal. Eastern coals average about 1.4 percent nitrogen, but with a range of 0.7 to 2.5 percent.
Metal values make up the part of coal commonly referred to as ash. They are found in coal as macroscopic and microscopic particles as discrete particles, cavity fillings, and aggregates. Concentration ranges from a few percent to 15 or 20 percent.
Physical separation of these three constituents from coal is not satisfactory, as at best only a portion of them are removed. Furthermore, flue gas scrubbing is not entirely satisfactory as a means for sulfur and hazardous metals removal, as at the present stage of development such systems (primarily for sulfur emissions control) are only about 75% efficient, large quantities of sludges are formed which present a disposal problem, and the cost for flue gas scrubbing is high. Since the quantity of low-sulfur coal is limited and coal is our major source of energy, new or improved technology must be developed for cleaning coal prior to combustion to supply the United States with a clean coal and at the same time reduce the pollution of our environment. We have discovered that the majority of the sulfur and much of the ash including such toxic or hazardous metals as beryllium, boron, and lead can be extracted directly from the coal by treatment according to the present invention.
Previously proposed desulfurization processes have placed major emphasis on (1) the use of alkali and alkaline earth compounds at temperatures above the melting point of the compounds or at temperatures where the solid carbonaceous materials begin to decompose, (2) the use of steam or steam and air at slightly elevated temperatures, or (3) the use of high temperature (approximately 1000.degree. C) in atmospheres of such gases as nitrogen, carbon monoxide, and methane. A number of patents teach the use of sodium hydroxide, calcium hydroxide or mixtures thereof at temperatures above the melting point of these materials. In some cases the reagents are added to the solid carbonaceous materials as aqueous solutions. However, the water is volatilized during desulfurization at the elevated temperatures. Other patents disclose the use of gases such as steam, nitrogen, hydrogen, hydrocarbons, carbon monoxide and ammonia, or mixtures thereof, at elevated temperatures to desulfurize solid carbonaceous materials.
In comparison with these processes, for example, there is no need, and in fact it is not desirabble, in the present invention to first solubilize the coal in order to extract the sulfur and ash constituents. Furthermore, the present invention provides superior results and advantages with solid carbonaceous fuel that would not be expected from the prior art relating to treatment of liquid coal extracts.
Reggel, L., Raymond., R., Wender, I., and Blaustein, B. D., in their article, "Preparation of Ash-Free, Pyrite-Free Coal by Mild Chemical Treatment" Preprints, Division of Fuel Chemistry, ACS, V. 17, No. 1, August, 1972, pp 44-48, discuss the removal of pyritic sulfur from coal by treatment with a 0.10 N aqueous solution of either sodium hydroxide or calcium hydroxide individually for 2 hours at a temperature of 225.degree. C. However, they do not discuss treatment with a mixed alkali solution, nor do they recognize the unique benefits arising from such treatment. More particularly, we have discovered, and they fail to recognize, that treatment with a mixed alkali solution according to the present invention results in: (1) the removal of a substantial amount of the organic, as well as the pyritic, sulfur from the coal, thus generally resulting in a coal having a lower total sulfur content than coal treated according to Reggel, et al.; (2) an unexpectedly great increase in the gasification reactivity of the coal; (3) an unexpectedly great decrease in the sodium content of the coal; and, (4) generally, a decrease in the required length of the treatment time.